Generally, rubber modified styrenic resins can have good mold processability and mechanical strength and have accordingly been widely used in the production of internal or external parts of electric/electronic goods and office equipment.
However, rubber modified styrenic resins can readily catch on fire. Accordingly, rubber modified styrenic resins are subject to various mandatory controls on flammability for safety reasons in the United States, Europe, and other countries, and are required to have high flame retardancy to meet the Underwriter's Laboratories Standard for use in the housing of electric appliances. Therefore, efforts are on-going to improve the flame retardancy of rubber modified styrenic resins.
Widely known methods for imparting flame retardancy to rubber modified styrenic resins include the addition of halogen-containing flame retardants, phosphorus-containing flame retardants, and inorganic flame retardants. Flame retardant aids may also be employed in combination with flame retardants to further increase flame retardancy.
Currently, electric/electronic products and office equipment are becoming larger overall, yet thinner. When rubber modified styrene resins are employed in such a large, yet thin, product, it can be necessary to add a large amount of flame retardant to obtain sufficient flame retardancy. As a result, impact resistance and rigidity of rubber modified styrene resins may decrease.
Polyester resins typically have a structure including short chains, and thus may not bend easily. Accordingly, polyester resins can have good rigidity, electrical properties, weather resistance, heat resistance, and the tensile strength of polyester resins may not be affected even after long term exposure to high temperatures. Further, crystalline polyester resins have good resistance to various oils, such as diesel oil.
However, the properties of polyester resins can degrade when they are exposed to acid or alkali for a long time at a high temperature due to ester bonds in the polyester chain. Thus, reinforcing agents such as glass fiber can be added to the polyester resins when polyester resins are employed as a structural material. If reinforcing agents are not added, it is difficult to use polyester resins as a structural material produced by injection molding.
It can be particularly difficult to impart flame retardancy to polyester resin because of flame dripping due to very rapid degradation of the polymer during combustion. Accordingly, in order to obtain flame retardancy, reinforcing agents such as glass fiber are added to polyester resin, because polyester resin alone is not flame retardant.
Recent attempts to impart flame retardancy to polyester resins have included alloying polyester with rubber modified styrene resin. The impact resistance of the alloy, however, is significantly decreased. Hence, the alloy cannot be used as a structural material.